Hi all, so in the end i manage to correct this issue.
We had to remove zwift from both of out laptops, making sure to delete the Zwift folder in your Documents.
We then reinstalled on both computers.
We then connected each computer to our devices one at a time making sure all of the other persons devices were switched off. From that point forward we have never ever swapped computers or connected to anyone elses kit ourselves.
When we get on the bikes now, we wake up all devices on both bikes and then start up zwift, it automatically sees the devices and goes straight past the pairing screen with the little countdown timer thing.
Up to now neither computer has taken over the others trainer.
I dont know what part of this corrected the issue, nor am i skilled in the arts of computer magic, but so far so good.
Oh yeah, i got the reccomended extension cable for my ant dongle, and now have it cable tied to my seat stay above the trainer. Again no idea if this has actually helped.